Extracorporeal circulation is and has been a routine procedure in the operating room for several years. An important component in the extracorporeal blood circuit is a heat exchanger used to lower the temperature of the blood prior to and during a surgical procedure and subsequently rewarm the blood to normal body temperature. The cooled blood induces a hypothermia which substantially reduces the oxygen consumption of the patient. The published literature indicates that the oxygen demand of the patient is decreased to about one-half at 30.degree. C., one-third at 25.degree. C. and one-fifth at 20.degree. C. Light (33 to 35.degree. C.), moderate (26 to 32.degree. C.), and deep (20.degree. C. and below) hypothermia are commonly used in clinical practice. Hypothermia is used to protect the vital organs including the kidneys, heart, brain and liver during operative procedures which require interrupting or decreasing the perfusion.
A number of different structural configurations for heat exchangers have been used in the extracorporeal blood circuit including hollow metal coils, cylinders and plates through which a heat transfer fluid (typically water) is circulated. A survey of a number of different type of heat exchangers used in extracorporeal circulation is included in the book entitled "Heart-Lung Bypass" by Pierre M. Galletti, M.D. et al, pages 165 to 170.
Notwithstanding the plurality of different types of heat exchanger configurations which have been used in the past, there remains a need for a safe highly efficient heat exchanger design which is simple to use and yet inexpensive enough to be manufactured as a disposable item. Thus, it is important that there not be any leakage of the heat transfer fluid into the blood. This fluid is typically circulating water flowing from plumbing fixtures located in the operating room. Certain of the heat exchangers commonly used today for clinical bypass operations have an upper pressure limit which is sometimes lower than the water pressures obtainable in the hospital operating room. The person who connects up the heat exchanger must therefore be very careful to never apply the full force of the water pressure through such a heat exchanger. Failing to take this precaution, or an unexpected increase in water pressure, could cause a rupture within the heat exchanger resulting in contamination of the blood flowing through the blood oxygenator.
It is also important that the heat exchanger have a high performance factor in order to reduce to a minimum the time required to lower the temperature to induce hypothermia and subsequently raise the blood temperature to normal. Some physiological degradation of the blood occurs after a patient is connected only a few hours to any of the bubble oxygenators presently in use. Therefore, time saved in cooling and rewarming the blood is of direct benefit to the patient and also gives the surgeon additional time to conduct the surgical procedure if necessary. Heretofore, stainless steel tubing has ofter been used in blood environments as a heat exchange element because it is biologically compatible with human blood. However, heat exchangers made with stainless steel tubing are very expensive and are not economically suited for a throw away disposable unit.